This happens to me on Listpro if I modify a document on the phone and modify it on the computer and then sync. The computer doesn't know what to do since you have modified both and its not sure which ones to replace. I click on the "attention required" notification and it will try to resync again with the same issue, after 2-3 times of doing this it will finally ask me which one would you like to replace (desktop or handheld version) and then you decide and that will be it, it will copy the file. Another idea can be to reinstall activesync if you can't fix it.